1. Field of the Invention
This invention lies in the field of centrifugal gravity separation apparatus, especially cyclones for specific gravity separation of solid particles and methods of using such apparatus for the cleaning of sized and crushed coal and similar ores wherein the cleaning liquid is water which is mixed with the coal or the ore. Control of velocity and solids content separates the crushed particles according to gravity, the lighter particles constituting the purified coal in the case of coal washing escaping through the cyclone box at the top and the heavier gravity particles constituting rock, ash and other impurities falling out at the bottom. The cyclones may be placed in several groups in series or in parallel.
2. Description of the Prior Art
(A) Economic Background
Coal is consumed in greater tonnage than most commodities produced by man, annual production in 1964 averaging about 400 million metric tons in the United States and 2,600 million metric tones for the entire world. Coal reserves far exceed the known reserves of all other mineral fuels (petroleum, natural gas, oil shale, and tar sands) combined in the United States and for the world as a whole. Since the supplies of other fuels, especially oil, are now becoming depleted and as industrial growth continues, coal is once again being used in larger and larger amounts.
The need for washing or cleaning coal has long been recognized. Coals of high and medium rank as classified by ASTM (see Kirk-Othmer, Encyclopedia of Chemical Technology, Vol. 5, 1964, page 651, John Wiley and Sons) must be sized and washed to meet industrial specifications.
In England in 1934 there were 611 washeries and 150 dry cleaner plants to handle only 40% of the output. Coal at that time was generally hand picked and hand washed. Only by the considerable efforts of the Ministry of Fuel and Power was sizing introduced in England. For industrial use sizing, it was pointed out at page 22 of the text "The Efficient Use of Fuel" by John Olsen, Chemical Publishing Company, 1945, that:
(1) sized coal avoids segregation and promotes efficiency of end use;
(2) if coal is not sized, there is uneven combustion and loss of energy when using automatic stokers;
(3) sized coal is a requirement in gasification, metallurgy and steam generation;
(4) sizing is an important asset in power generation by producing a fuel bed of uniform resistance to passage of air or gasification.
Even though coal was known and used by the ancients in Greece, Italy and China more than 2,000 years ago, it is only in the last 25 years that mining, preparation, storage and transport have all become more or less mechanized. Of these, the greatest progress in automation has been in mining. The least progress has been in coal preparation, e.g., in mechanical or chemical cleaning compatible with the regulations to protect the environment, especially the air pollution requirements and the more stringent demands made by industrial customers for maximum efficiency of coal product and cost effectiveness of preparation procedures. Lack of progress is not due to the poverty of ideas or washing or cleaning methods or systems which include:
(1) jig washing PA1 (2) heavy media washing using expensive magnetite PA1 (3) trough washing PA1 (4) use of washing tables PA1 (5) cleaning of coal dust PA1 (6) froth flotation using chemical flotation or frothing agents.
None of the above systems permits a single washing station to handle the output of a large mine or a number of small mines and thereby reduce costs. Each of the above methods achieves quality product at high cost. It is the high cost to which the present simple, automatic operation of the invention provides an entirely new approach and an environmentally satisfactory solution.
(b) Prior Chemical Machinery Used in Coal Grinding and Washing
In the well known text by Ernie R. Riegel entitled Chemical Machinery published by Reinhold Publishing Company, 330 West Forty Second Street, New York, N.Y. (1944) at page 21, there is shown the commercially available standard double roll crusher which serves to produce the raw crushed coal for the coal pile which is the starting material after it is sized. At page 53 of this text there is shown the screen machine which sizes the crushed coal, larger pieces being returned to the crusher. The screen machine preferred for coal is a one surface Tyler-Niagara screen machine or equivalent which produces a product of 3/4".times.0.
Belt conveyors of known type as shown at page 79 of the text bring screened coal into a conventional slurry tank which is fed with water in a separate line to proportion solids to between 18% and 50% by weight. This is all the mixing which is required before entering the pump, which is a conventional centrifugal pump of the volute type shown in FIG. 95 at page 121 of the text.
These preliminary steps are the prologue to the invention.
(c) Prior Patents
Fitch, U.S. Pat. No. 2,981,413 dated April, 1961, proposed the use of a vortex finder as classifier means in a large capacity cyclone for the separation of fines from coarse particles in a process of separating solids in liquid suspension. The vortex finder is identified by reference 21 in Fitch and is described at column 6, line 63, as a withdrawal conduit adapted, by virtue of its placement along the central vertical axis of the cyclone, for continuous withdrawal of fines in the overflow at an accelerated rate relative to the rate of removal of coarse particles at the bottom. Fitch classifies and separates fines from coarse and shows the location of adjustment of the bottom of the inner vortex finder relative to the conical bottom of the cyclone for best separation and also shows a fixed location of an outer vortex finder. Fitch's objective in using two vortex finders was to eliminate energy loss due to turbulence (see column 8, lines 51-52). Because of large capacity Fitch had to live with turbulent flow.
Visman, Reissue 26,720 dated November, 1969, was the first to realize success in keeping size separation, as in Fitch, to a minimum while achieving gravity separation using finely crushed coals 1".times.0. Visman's examples are all of 1/4".times.0 at low pulp solids of about 10%. Visman contemplated cyclone diameters as large as 24" and as small as 2". Visman described a cyclone capacity of 70 tons per hour at relatively high velocities above 6 psig.
Gay, U.S. Pat. No. 3,926,787, used much coarser crushed coal than Visman, e.g., 11/4".times.0, and taught substantially higher coal solids in the pulp, at column 9, line 19, with the optimum at 15% for strip mine coal. Despite the higher coal solids in the pulp and his stated objective of maximum capacity for washing purification, e.g., a value of 48 tons/hour on a dry weight basis (see column 9, line 26). It was clearly stated by Gay that both size and gravity separation were achieved under his turbulent mixing and operating conditions. Thus Gay achieved washing purification at about the same pulp solids as Visman without in any way dealing with the loss of energy due to turbulence which was described earlier by Fitch. While Fitch added a second vortex finder to diminish turbulence once started, Gay created turbulence in his special mixing chamber which delivered pulp to the cyclone.
Other patentees have proposed deflection surfaces and deflectors, for example Prins et al, U.S. Pat. No. 3,288,286, of November, 1966, and Hebb, U.S. Pat. No. 2,161,563, of November, 1952.
Prins et al shows a deflection surface which constricts incoming flow but there is no placement of the constriction to divert all of the particles to the single inlet tangential wall of the cyclone.
Hebb shows two deflectors 22 and 23 within the bowl itself, these starting at the end of the inlet pipe. Hebb separates by size and not by gravity (see column 5, lines 64-75). The deflectors or vanes control radial speed by restricting flow and directing the particles to the inner wall or box of the cyclone, thereby creating turbulence due to the reverse travel of the particles to the outer wall of the bowl.